Racket string



A. F. SMITH RACKET STRING May 28, 1946.

. Filed June 11, 1943 IN VEN TOR A1. BERT/T5M/7H ATTORNEY Patented May 28, 1946 RACKET s'mmc Albert F. Smith, Westwood Manor, Wilmington, Del., assignor to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware Application June 11, 1943, Serial No. 490,438

2 Claims.

This invention relates to coated nylon cords. More particularly, it relates to the coating of cords of multifilament nylon yarns t produce resilient, tough, high-impact strength, abrasionreslsting structures for use as athletic racket strings,

Although the cords of the invention are suitable for all uses where toughness, resilience, high-impact strength, abrasion-resistance, etc., are demanded, the invention will be discussed in terms of athletic racket strings, and particularly tennis racket strings, for convenience.

Tennis rackets strung with single strand nylon monofils, while vastly superior in most respects to rackets strung with gut, have had certain inferior playing qualities and other disadvantages attendant upon the use of such single strand nylon monofils. The disadvantages are apparently due to the physical structure and forma tion of the monofils. It has been found that Iangitudinal splitting of the monofils occurs after a certain length of time. The splitting is regarded as being due to the lowered transverse strength because of the high longitudinal orientation of the monofils. The splitting results from nicks which are caused by the contact of the taut string with the dirt and grit carried by the ball. In stringing a racket, the longitudinal strings are drawn across the transverse string with a force approximating 80-100 pounds to achieve the necessary final high string tension. The nylon monofil strings formerly used have been known to become weakened and to eventually break in play because of chipping orwearing at points where the strings cros each other. Thi failure may be caused by brittleness, surface flaws, the absence of symmetry throughout the length and structure of the filament, or grooving of the transverse filaments in stringing.

It is an object of this invention to produce nylon tennis racket strings superior to former nylon tennis strings and devoid of the abovestated objections.

It is a further object to produce nylon tennis strings which possess all of the desirable properties of the nylon of former tennis strings and, in addition, that elusive quality known as good playing characteristics.

Other objects will appear hereinafter.

The objects are accomplished by forming a cord of twisted nylon multifilament yarns and coating it to product a tough, seamless skin on the twisted cord structure.

Nylon tennis racket strings have heretofore been made exclusively of singl strand monofilamentary yarn since nylon multifilament yarns were considered obviously and wholly unsuited to such usages because of their ease of stretchability, their lack of requisite stifi'ness or body, as well as the extreme sensitivity of the exposed filaments to abrasive action.

An important discovery of this invention rests in the fact that multifilament nylon yarns, when corded to produce a balanced cord and coated, make entirely acceptable and even superior tennis strings. These new tennis strings inherently possess the resiliency and recovery from stretch of twisted yarns, the iron-like strength of nylon yarns and the abrasion-resistance afforded by the coating which are important prerequisites for satisfactory tennis strings.

The present invention is illustrated in the accompanying drawing forming a part of this specification and wherein:

Figure 1 is a top plan view of a tennis racket strung with strings embodying the principles of the instant invention;

Figure 2 is a cross-section of a string shown in Figure 1;

Figure 3 is an enlarged and exaggerated view of one embodiment of a string, the coating being omitted, and partly raveled to show th construction thereof; and

Figure 4 is an enlarged and exaggerated view of another embodiment of a string, the coating being omitted, and partly raveled to show the construction thereof.

Referring now to the drawing, the reference numeral I designates a tennis racket frame strung with strings 2. Each string. 2 consists of a cord 3 obtained by twisting together a plurality of strands 4, and each strand 4 is obtained by twisting together a plurality of nylon filaments 5. Th cord 3 is provided with a coating 6.

In order that the invention may be more clearly understood, the following are examples of the method of construction of these nylon tennis strings:

Example I 210-denier, 68-filament, l-turn S polyhexamethylene adipamide yarn 5 was treated in the relaxed condition with boiling water as i known in the art to increase the impact strength. Six ends of the yarn 5 were twisted together 6.2 turns 8 to yield the strand 4. Six ends of the strand 4 were then twisted together 2.? turns Z to form the cord 3. The cord 3, which weighed 0.85 gram per yard. was coated with a nylon solution to build up a coating 6 weighing approximately 0.37 gram'per solids content. The coating solution was appliedat the speed of 8 feet per minute and loose felt wipers on the, cord removed the excess coating. The coating was dried at about 90C. in a 12-foot oven and one dry pass was used after the -application of the last coat.

The cord so formed and'which wa composed of about 30% coating was then strung in a tennis racket frame and, after being experimentallyused' and tested'forover a period of a year under conditions of actual use, the racket exhibited'exceptionally good service and was considered superior to former nylon monofil strung rackets.

Etcample- 'I I' 30-denier filaments i of polyhexamethylene adipamide were treated with aqueous phenol as is understood in the art to improve the impact strength. Thirty-six ends. were twisted turns s and six ends of the resulting strand ,4 were.

twisted 4 turns Z to form the cord 3'. The corded structure ,was coated with N -methoxymethyl polyamicle,v made ff rom polyhexamethylene adipamide,-in an ethyl alcohol solution solids .content).. By successive coatings, a covering of about 28% of;the final .structure was produced.

The coated structure wasthen bakedat 120 C. for-3 minutes. When strung in atennis racket in the usual way, it was found to have excellent playing characteristics and,very long life.

The multifilament nylon; yarns which can be used in this invention are not limited to those disclosed in the foregoing examples .but include also all filaments comprisingfiber-forming synthetic linear polymers, especially those of the classes described-in U,S.;Batehts1l los. 2,071,250, 2,071,251, 2,071,253, 2,130,523, and 2,130,948, as well as those filaments comprising .the interpolymers of. U. S. Patents-Nos. 2,252,554 and 2,252,555. In other words, the invention is intended to include for the preparationof the filaments all water-insoluble and fiber-forming nylon superpolymers and interpolymersr ;The method of twisting and stranding the filaments given in the examples is merely illustrative of conventional-procedures used in making cords for tennisrackets,.andthe invention isnot meant to be limited thereto since. it is obvious-that there can bensed other. combinations of twists, number of ends for building up a cord, amount or direction .of twist of either the strand or cord or both, to produce a final cord. structure having the proper .balance to produce resilience, impact strength and abrasion-resistance.

As shown by the examples, the coating composition will preferably comprise any of the synthetic linear polyamides capable of forming coating compositions, such as, for example, the polyamidesand interpolyamides of the type mentioned in U. S. Patents Nos. 2,071,253 and 2,130,948.

The invention. is not: limited, however,- to the use of synthetic-linear polyamides as coatin compositions. Other coating compositions which produce well-adhering, tough, abrasion-resisting, flexible and fatigue-resisting covering can be used. Examples of other materials which may be used are polyesters,-polyacetals, polyesteramides,

polyurethanes, polythioureas and p ymeric ethylene. Properly plasticized polymeric methacrylates. polymeric acrylates, polyvinyl'butyral, vinyl chloride vinyl acetate interpolymers', alkyd resins, heat-hardenable phenol-formaldehyde resins, and cellulose derivatives or suitable mixtures of the above resins are also applicable.

While the exampleshave shown the coating to be approximately 30% of the cord structure, the coating may constitute -40% of the final cord structure without departing from the spirit and scope lof the invention.

e coating may be applied by any of theconventio'nal eoatingmethods, i. e.. spr y ppin extrusion; dispersion, etc. While a preference 4 has' been shown for the application of the coating by solvent methods, the invention-is not so limited.- Any technique which results in a uniform,

continuous. adherent coating is satisfactory. The tennis racket strings of this invention have exceptionally good playing characteristics.- Because of their structure, the strings possess greater resilience and flexibility, higher tensile and impact strengths, more highly abrasiveresisting coatings and longer playing life-than have been achieved heretofore. These-strings .also possess apronounced fatigue-resistance,

holding approximately the same string,- tension after a year of use. The strings of the invention when the string is pulled through sharp bends to prevent ,the loss of any of the desirable playing .characteristics or durability inthe. stringing operation.

The improved strings of ,this invention are, especially valuable in athletic rackets, suchas tennis, badminton, squash racketsand the like. 40 --f Since it is obvious that various changes. and modifications may be made in the above description without departingjfrom the nature: or spirit thereof, .this invention is not .restricted thereto except as set forth in the appended claims.

said' coating giving sufllcient protection-against abrasion when the string is pulled through sharp bends to prevent the loss of the desirable playing characteristics or durability in the stringing operation', said corded structure comprising-a plurality linear polyamide prepared fromhexamethylenediammonium adipate, hexamethylenediammm niuni sebacate andcaprolactam inthe ratio of 40:30:30, said coating iving sufllcient protection against abrasionwhen the stringis pulled through sharp bends to prevent the loss of the-undesirable playing characteristics orrdurability iii-the stringing.operation,'said corded structure comprising a plurality of strands twisted together, each strand comprising;a,plurality. of polyhexamethylene adipamidefilaments twisted together,

said coating constituting 20% to 910% by weight of said string.

ALBERT F. SMITH. 

